1. Field of the Invention
The present invention relates to a rubber composition, more specifically, the invention relates to a rubber composition exhibiting excellent adhesion and heat aging resistance when used as rubber for coating steel cords.
2. Related Art Statement
Recently, there have been remarkably developed so-called steel tires using steel cords in belts and/or carcass plies, and various investigations have been made on the adhesion between the steel cords and the rubber.
In general, the adhesion between the steel cords and rubber is broadly classified in two ways, that is, a method of an RHS adhesion system with resorcinol/hexamethylene tetramine/silica, and another method of blending a metal salt, particularly a cobalt salt of an organic acid, have been known. The latter method has been more widely used, and the rubber compositions blended with such a metal salt have been proposed as the steel cord-coating rubber. For instance, Japanese Patent Publication No. 56-39,828, Japanese Patent Application Laid-open No. 54-52,188, Japanese Patent Publication Nos. 50-33,905, 49-20,072, 52-26,276, and 54-31,511 and Japanese Patent Application Laid-open No. 58-89,631 disclose rubber compositions blended with cobalt naphthenate, cobalt stearate, cobalt oleate, cobalt salt of tall oil acid, cobalt resinate and the like as an adhesive promotor.
In the meanwhile, the demands for the safety, high speed running performance, durability and so on for the automobile tires have been getting larger and larger, and the conventional techniques have not necessarily satisfied such demands. For instance, the cobalt salts blended into the rubber compositions promote the heat aging of the rubber, and among them, cobalt naphthenate extremely decreases the heat aging resistance. Further, cobalt resinate lowers the tackiness in an unvulcanized state, so that the adhesion between the rubber members becomes poorer and problems of the poorer adhesion rises after the vulcanization. In addition, cobalt resinate is generally excellent in the heat aging resistance as compared with cobalt naphthenate. However, upon having made investigations, the present inventors have found that all the cobalt resinates do not necessarily exhibit good performances; some rubber compositions even blended with cobalt resinates exhibit poor adhesion and particularly poor initial stage adhesion, which is the largest shortcoming of the cobalt resinates, and they are not satisfactory in terms of the tire durability.